Rising fuel costs and concern for the environment are providing more and more incentive for automobile and heavy duty vehicle manufacturers to increase the fuel economy of the vehicles they produce. New vehicles are including new and improved technologies that continue to reduce the amount of fuel used per mile driven. One such technology is the hybridization of vehicles in which two or more power sources are used in various combinations to propel the vehicle.
Depending on the exact method of hybridization of a vehicle the fuel savings may be realized from one or more of the following: optimizing the operating speed and load point at which the prime mover or engine operates, recovering braking energy normally lost to heat, increasing the efficiency of the powertrain, enabling stop-start operation, and by allowing for engine downsizing.
Electric hybrids utilize an internal combustion engine in combination with one or more electric motors. Similarly, hydraulic hybrids utilize hydraulic pumps and motors as a second source of power and store the energy as pressurized fluid in accumulators. Hydraulic systems have higher power density than electric systems, making hydraulics ideally suited for vehicles intended to recover large amounts of braking energy. Typical examples of vehicles that could benefit from hydraulic hybrids include refuse trucks, delivery vehicles, or other vehicles that undergo frequent stopping and starting.
Researchers and suppliers have been developing both parallel and series hydraulic hybrid systems for some time, but many of the limitations of these systems have yet to be overcome. Parallel hybrid systems can typically be characterized as simpler systems that rely largely on regenerative braking to generate a majority of the fuel savings, which tends to be considerably less than those achieved with series systems. Series systems tend to be more complicated, cost more as a retrofit, but save larger amounts of fuel largely due to their ability to decouple the engine load from the required driving load. Finally, series hydraulic hybrid systems tend to be more difficult to make financially viable for retrofit applications due to the large amount of hardware needed for the system.
Most parallel hydraulic hybrid systems are installed in the driveline downstream of the transmission resulting in multiple disadvantages. The first disadvantage is that by locating the pump/motor downstream of the transmission the pump/motor does not get the benefit of the torque multiplication of the transmission, and more importantly the ability to easily employ multiple ratios. This requires that the system either incorporate large gear reductions, thus limiting the vehicle speeds at which the hybrid system can be operated, or that the system be designed to use larger pump/motors which are larger, heavier and more expensive than smaller pump/motors.
Secondly, installing the system after the transmission requires that the driveline needs to be modified in the case of retrofits and makes integration more difficult in new designs.
Additionally, the larger pump/motors required in the driveline systems are heavier, require more space which can be in very short supply, and tend to cost more due to increased materials cost and reduced economies of scale associated with lower production numbers. There is a compounding affect as the pump/motors get larger, because the bigger pump/motors will require more fluid which results in larger reservoirs with more oil in them. Furthermore, the heavier pumps and gearboxes and larger reservoirs will require heavier mounts and reduce the vehicle payload and fuel savings.
Finally, one of the largest shortcomings of existing parallel hydraulic hybrids is that they are designed to be integrated primarily into vehicles that utilize automatic transmissions. Vehicles with automatic transmissions have poorer fuel economy than manual transmissions do in almost all cases. This decreased efficiency comes primarily from the use of a torque converter that is known to be very inefficient, especially during launch. A “torque converter” is a hydrodynamic torque converter that is commonly used to couple automotive and heavy truck engines to automatic transmissions. Many newer transmissions now utilize a torque converter clutch (TCC) to lock up the converter, thereby eliminating most of those losses in some of the operating regions, but the TCC cannot be applied during vehicle launch (a time during which there is a large amount of power loss). While a TCC can increase efficiency and reduce the fuel consumption, the overall efficiency is still usually lower than that of manual transmissions.